When I was a kid, I had a friend whose birthday was on February 29. I used to rib him that he was only 3 years old, and he would visibly restrain himself from punching me. Evidently he heard that joke a lot.
Of course, he was really 12. But since February 29 is a leap day, it only comes once every four years.
But why is leap day only a quadrennial event?
Why is anything anything? Because astronomy!